The present invention relates generally to a method of controlling the movement of a tool in the process of machining a block of material to produce a work piece of a prescribed shape, where a working portion of the tool that serves to shape the block by removing material from its surface, moves along a guide path extending in an essentially planar guide surface, so that an axis of the tool intersecting the guide path maintains a given orientation in relation to the guide surface.
There are known various methods used, in particular, of producing complex-shaped work pieces in the fabrication of tools and dies. The material block from which the work piece of the prescribed shape is to be formed, is machined by a cutter head of a three- to five-axis milling machine. An optimization of this work process contributes significantly to a cost-effective production of sheet-metal and plastic parts of the kind used, for example, in the automobile industry and aircraft industry, as well as in the consumer goods industry.
However, the disadvantage of the conventional methods is that the work pieces that tool and die makers are required to produce are of increasingly curved and complex shapes that are difficult to achieve by conventional milling techniques. In particular, the milling of cavities, i.e., deeply recessed, hollowed-out spaces in the work piece, require a 3+2-axis milling process where the conventional practice is to set the cutting head at an acute angle-to the rotary axis of the actual milling tool, at least along sections of the guide path. However, this creates the difficulty that the cutter head could collide with the shape of the work piece that is in the process of being formedxe2x80x94a risk which is a significant drawback with these conventional methods, and against which these conventional methods do not offer sufficient protection.
Further, another disadvantage to the conventional methods, is that cavities can be produced only to a very limited extent.
It is therefore a feature and advantage of the present invention to provide a method of controlling the movement of a tool in the process of machining a block of material to produce a work piece of a prescribed shape, which allows work pieces of highly complex shapes to be produced with a high degree of process reliability.
In accordance with a first embodiment of the present invention, a method is disclosed which includes guiding a point on the tool axis located at a distance from a first guide path along a prescribed second guide path. The present invention is particularly well-suited for generating programs for five-axis milling machines. However, the method of the present invention is not limited to this particular application, but is also suitable for controlling the movement of other material-removing tools such as laser tools, for example.
In the first embodiment of the present invention, while the first guide path is selected so that the material-removing working portion of the tool (i.e., the cutting tip of a mill cutter), in moving along the first guide path which corresponds to a contour curve of the desired work piece shape in the guide surface, the second guide path is suitably selected so that the orientation of the rotary axis of the tool (i.e., the mill cutter), is at any time in the process optimally adapted to the surface wall of the work piece as it is being shaped, and/or the tool and its tool holder are held at a predetermined distance from the surface of the work piece.
As the shapes of the work pieces are customarily modeled in computer-aided design (CAD) systems, the CAD data can be provided through integrated or standardized interfaces as a basis for the computation of guide paths. The guide paths computed from the CAD data are delivered in data formats that are compatible with computer numerical controls (CNC""s). The CNC""s will then guide the numerically controlled machine tool so that two predetermined points on the tool axis, follow the first and second guide paths, respectively.
Therefore, the machine tool can be a five-axis milling machine which allows translatory movement of the milling tool along three mutually orthogonal translatory axes, as well as swivel movements about two mutually orthogonal rotary axes.
The computed guide paths need not be exactly planar curves (i.e., the guide surfaces containing the guide paths do not have to be perfectly planar). Further, it can be seen that in the course of machining a material block, the tool moves along a multitude of guide paths that run in guide surfaces which are, for example, mutually parallel, and also, for example, spaced apart at a perpendicular distance from each other. For each first guide path extending in a given guide surface, the associated second guide path is determined by the CAD system. The first guide paths, are in effect, analogous to the contour lines of a topographical map representing the shape of the work piece.
It should be noted that, according to known computing algorithms, the shape of the work piece and the guide paths need to be modeled only as accurately as required in accordance with given tolerances, so that the known techniques of approximating work piece surfaces and contour curves through trimmed surfaces, solid geometries, or approximation curves, can be used.
In a preferred embodiment of the present invention, the orthogonal distance of the second guide path from the guide surface containing the first guide path, is kept constant. For example, the guide surface containing the first guide path may be parallel to the xy-plane of a spatially fixed coordinate system, meaning that only x and y are variable along the first guide path, while the z-coordinate of the first guide path has a constant value. The surface containing the second guide path runs parallel at another fixed value of the z-coordinate.
In a preferred embodiment of the present invention, the second guide path is defined as the locus of all points that are obtained by transposing each point of the first guide path in a direction orthogonal to the first guide path, to a point located at a given distance parallel to the guide surface, and another given distance perpendicular to the guide surface. Consequently, if the second guide path was projected perpendicularly onto the guide surface of the first guide path, the first guide path and the projection of the second guide path would run within the essentially planar guide surface at a constant distance from each other. In the case where the first guide path forms a closed loop, the projection of the second guide path will run inside the first guide path, if the corresponding surface detail of the work piece is of concave shape. The projection of the second guide path will run outside the first guide path, if the corresponding surface detail of the work piece is of convex shape.
In another feature of the present invention, for each point where the axis of the tool intersects with the first guide path, the associated point where the tool axis is guided on the second guide path is determined as the point of minimal distance from the point of the first guide path. This assures the synchronous progress of the point traveling along the second guide path with the advancement of the point of the tool that is guided at a given speed along the first guide path. The parametric representation of the first guide path as a function of time, which determines the speed of motion of the point on the tool axis, provides a simple means of computing the synchronous parametric representation of the second guide path. As a result, the two points on the axis of the tool, and thus, the orientation of the tool axis, are unambiguously determined.
In this context, as a further advantage, the point of intersection of the axis with the first guide path moves along the latter at a uniform speed, and the second guide path is a suitable assembly of approximation curves so that the point of the axis moving along the second guide path advances smoothly (i.e., relatively free from joltsxe2x80x94which are defined as the first derivative of acceleration (i.e., the third derivative of the travel path with respect to time)). While the given speed assures a jolt-free movement along the first travel path, the resulting second travel path may have points where a time derivative is not defined; a condition which would cause undesirable jolts when the tool axis passes through the respective locations on the second travel path. Through an appropriate selection of the approximating curve segments (i.e., cube spline functions), the jolts can be minimized by assuring the continuity of the third derivative with respect to time.
Another important feature of the present invention is that for every tool position of the tool on the first guide path and the associated orientation of the tool axis, the positional relationship between the geometrical envelope of the tool and the topography of the work piece surface is calculated in advance. In the case that a spatial interference is found in a place other than the working portion of the tool, the present method includes the step of predicting a collision between the two spatial envelopes. A collision between the tool (i.e., the milling head consisting of the cutter and the tool holder), and the work piece surface in progress, can be avoided by being circumspect in the selection of the second guide path.
Another important feature of the present invention is automatically predicting and avoiding a collision. This takes into account not only the geometrical shape of the tool itself, such as an elongated cylindrical cutter, but also the shape of the tool holder, which in the case of deep cavities, has to enter so far inside the spatial perimeter of the work piece that it could collide with the walls of the cavity.
Based on this capability of predicting an impending collision, a preferred embodiment of the present invention provides an automatic collision avoidance by calculating the maximum angle (just short of the angle where a collision would occur) at which the axis of the tool can be inclined in relation to the perpendicular direction of the guide surface of the first guide path, and by setting the orientation of the axis according to the calculated maximum angle. With this reorientation of the axis, the tool can move on without any interruption of the work process.
As the reorientation of the axis can cause a change in the position of the working portion of the tool in relation to the material block being processed, a second embodiment of the present invention provides for a recalculation of the first guide path, taking the geometry of the working portion of the tool into account.
It is of significance that in this calculation, the geometry of the tool is substituted by an approximation surface that radially surrounds the tool. The approximation surface can be defined (i.e., by patching together surface segments as is customary in CAD systems), by a synthesis of solid geometries or by using lattice cell structures, which significantly simplifies and shortens the calculation of the spatial interference, because the collision-avoidance computation can be performed at a relatively coarse tolerance level. Given that the tools frequently are rotationally symmetrical in relation to their axes, and the tool holders in most cases have a larger diameter than the actual tool itself, conical approximation surfaces, in particular, would be a preferred choice (i.e., surfaces that extend from the working portion of the tool towards the tool holder with a widening conical taper to surround the tool holder).
There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract included below, are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.